In image formation according to an electrophotography system, an electrostatic latent image is formed on an electrostatic latent image bearer formed of, for example, a photoconductive material, a charged toner is deposited on the electrostatic latent image to form a toner image, the toner image is transferred onto a recording medium, followed by fixing, to thereby output an image. Recently, a technology related to photocopiers or printers using the electrophotographic system has been rapidly sifted from monochrome image formation to full-color image formation. A market for full-color image formation is expected to spread even more.
For the purpose of downsizing a fixing device for use, and simplifying a structure thereof, a toner containing a release agent is used in full-color image formation, similar to monochrome image formation. Therefore, an oilless system, where an oil is not applied to a fixing roller, tends to be employed. In full-color image formation, however, it is necessary to reduce a viscoelasticity of a melted toner in order to make a surface of a fixed toner image smooth. Therefore, offset tends to occur more easily than in a case of formation of a monochrome image having no gloss, and it is difficult to employ an oilless system in full-color image formation. When a toner containing a release agent is used in full-color image formation, moreover, adhesion of the toner increases to lower transferring properties of the toner to a recording medium. In addition, filming of the toner occurs to lower chargeability of a developer, to thereby decrease durability of the developer.
Regarding a carrier, meanwhile, a known carrier has a resin containing carbon black formed on a surface as a coating layer. Such a carrier is used for the purposes of preventing filming of a toner, forming a uniform surface of a fixed image, preventing oxidization of the surface, preventing reduction in moisture sensitivity, extending a service life of a developer, preventing deposition of a toner on a surface of a photoconductor, protecting a photoconductor from scratches or abrasion, controlling a polarity of charging, and adjusting a charged amount of a developer. Although excellent images can be formed using the aforementioned carrier at an initial stage, there is a problem that an image quality decreases due to peeling of a coating layer of the carrier, as the number of sheets copied increases. Moreover, there is a problem that color staining occurs because the coating layer is peeled off from the carrier, or carbon black is fallen off from the coating layer. In general, titanium oxide and zinc oxide are known as alternative materials of carbon black, but these materials demonstrate an insufficient effect of lowering volume resistivity.
PTL 1 discloses a carrier to which a coating layer containing antimony-doped tin oxide (ATO) as a needle-shaped conductive powder is formed. PTL 2 discloses a carrier to which a coating layer containing conductive particles is formed, and each conductive particle contains a base particle, and a tin dioxide layer and an indium oxide layer containing tin dioxide laminated on a surface of the base particle. PTL 3 discloses a carrier having a coating layer containing first conductive particles that are conductive particles of a metal oxide, and second conductive particles that are at least one of metal oxide particles and metal salt particles, where surfaces of the metal oxide particles and metal salt particles have been subjected to a conduction treatment.
Moreover, PTL 4 discloses a carrier having a coating layer containing barium sulfate. PTL 5 discloses a carrier having a coating layer containing a resin formed of a certain copolymer, and barium sulfate. Furthermore, PTL 6 and PTL 7 disclose carriers each having a coating layer, and disclose that a Ba/Si ratio is from 0.01 through 0.08 when the carriers are measured by X-ray photoelectron spectroscopy (XPS). Furthermore, PTL 8 discloses a carrier where barium sulfate is used as a base of conductive particles contained in the carrier.